Hybrid extrusion methods

ABSTRACT

A hybrid extruder provides plastic insulation which is as uniformly and tightly disposed about a substrate as that produced in a conventional pressure extruder and accommodates irregular substrates. The hybrid extruder includes a core tube which is positioned in a die cavity to form a flow passage which includes a restriction to the flow at the end of the core tube to maximize the pressure in the plastic material. The leading end of the core tube is spaced a distance from the land of a die which is substantially less than in conventional pressure extruders to form another portion of the flow passage having a predetermined configuration. After flowing through the restriction, the plastic material expands. This avoids any backflow of the plastic material into the core tube in the event the core tube is to allow the passage of oversized spliced portions of the substrate. Advantageously, this arrangement also prevents the occurrence of melt fracture when extruding relatively high molecular weight plastic materials which because of excellent mechanical and dielectric properties are desired for insulating particular substrates.

TECHNICAL FIELD

This invention relates to hybrid extrusion methods for insulatingsubstrates. More particularly, it relates to methods which differ fromconventional tubing or pressure extrusion techniques for covering aconductor or a plurality of conductors with a polymer material thatrequires relatively high pressures to extrude.

BACKGROUND OF THE INVENTION

The covering of substrates such as conductors or cores for use incommunications with plastic insulating or jacketing materials isgenerally accomplished with pressure or tubing extrusion tooling. Inpressure extrusion, a substrate is moved through a core tube having anopening that is only slightly larger than the substrate. The end of thecore tube is positioned within a die cavity and spaced from a land of adie through which the substrate and the plastic extrudate are moved.Pressure extrusion results in a well defined insulative cover which isdisposed tightly about the substrate.

In normal pressure extrusion tooling, the moving substrate is exposed toa relatively high melt pressure of the plastic material in a so called"gum space" between the end of the core tube and the die land. Flow ofthe plastic material is comprised of two components--differentialpressure flow and drag flow. The pressure flow is caused by thedifference in pressure between the entrance to the land and the exitorifice of the die. Drag flow is defined as the volumetric forwarddisplacement of a viscous material between a stationary and a movingsurface such as between the land and the substrate. See E. I. BernhardtProcessing of Thermoplastic Materials which was published in 1979 byKrieger Publishing Company.

This relatively high melt pressure requires that the inside surface ofthe core tube be only slightly larger than the outer dimension of thesubstrate. This voids any problems in concentricity of the insulationcover and creates a seal which prevents the extrudate from flowing in adirection opposite to the direction of advance of the substrate and intothe core tube. Typically, the clearance between the substrate's outersurface and the inner surface of the core tube is 0.001 to 0.002 inchfor product sizes in the 0.015 to 0.075 inch range.

Unfortunately, this relatively small clearance prevents any substrateirregularities such as intermittent oversize sections from passingthrough the tooling. Consequently, particular substrates having anon-uniform cross-section or any spliced, relatively smooth cores cannotbe insulated using conventional pressure extrusion techniques. If thecore tube is not oversized to accommodate these irregularities, thesubstrate will break, requiring downtime for operator string-up. On theother hand, if the core tube is oversized, the pressure in aconventional pressure extrusion process will cause a backflow of theplastic material into the core tube.

One such substrate having irregularities is that of a conductor of atelephone cord which is used with customer station equipment. Atelephone cord conductor generally comprises a polymeric core having aplurality of tinsel ribbons wrapped helically thereabout. Telephonecords are well disclosed in the prior art such as, for example, U.S.Pat. No. 3,037,068 issued May 29, 1962 in the name of H. L. Wessel, andin U.S. Pat. Nos. 2,920,351 and 3,024,497 issued on Jan. 12, 1960 andMar. 13, 1962 respectively in the names of E. C. Hardesty and D. L.Myers. Because a tinsel conductor is made with something less than aconstant cross-section, the core tube must be oversized.

For these kinds of products, the art has resorted to tubing processes inwhich the leading end of the core tube generally is flush with orextends beyond the die opening. See U.S. Pat. No. 3,554,042 which issuedon Jan. 5, 1971, in the name of E. R. Cocco. But in commonly assigned,copending application Ser. No. 229,434 which was filed on Feb. 29, 1981now U.S. Pat. No. 4,339,298, the downstream end of the core tube ispositioned within the die land. In a tubing operation, the clearancebetween the inner surface of the core tube and the outer dimension ofthe substrate, such as an array of tinsel conductors, is large enough topermit oversize substrate sections to be passed through the core tubewithout jamming. Unlike pressure extrusion, tubing relies solely ondifferential pressure flow and the extrudate is drawn down about thesubstrate externally of the die.

A tubing process does not always result in the most acceptable productsince tubed covers generally have more size variations and irregularsurfaces and are not disposed as tightly about the substrate as in apressure extrusion process. It should be clear that irregular orintermittently oversized substrates which are necessarily tube-insulatedor jacketed are done so at some expense to the overall productconfiguration and/or performance.

This disadvantage of a tubing process has been aggravated because ofrecent changes in the materials which are used for insulation andjacketing.

These changes in materials, at least for cords, have come about becauseof a somewhat recently introduced cord connection arrangement, which isreferred to as modularity. Miniature plugs are connected to each end ofa cord to facilitate attachment to jacks in telephone instruments and inwall outlets. For example, see U.S. Pat. Nos. 3,699,498 and 3,761,869issued Oct. 17, 1972 and Sept. 25, 1973 respectively in the names of E.C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr. and S. W. Walden andU.S. Pat. No. 4,148,359 issued Apr. 10, 1979 in the name of E. C.Hardesty. With the introduction of modularity, it became necessary touse a different core construction because of a need for a smallercross-section to be compatible with the plugs. In order to reduce thesize of the insulated conductor, the tinsel is insulated with acrystalline, relatively high molecular weight plastic material asdisclosed and claimed in U.S. Pat. No. 4,090,763 which issued on May 3,1978 in the names of W. I. Congdon, J. J. Mottine and W. C. Vespermanand which is incorporated by reference hereinto. A material such as thatdisclosed and claimed in the above-identified Congdon et al applicationis available commercially from E. I. duPont Company under the trade nameHYTREL® polyester elastomer.

Extrusion of the above-identified plastic material is characterized byrapid changes in melt viscosity and melt strength with slight variationsof polymer temperature. For relatively high molecular weight and/orbranched polymers such as HYTREL® polyester elastomer material, the meltviscosity increases significantly as the pressure increases. Thesecharacteristics could cause non-uniform wall thickness and polymer flowpulsations unless suitable control is exercised.

The prior art also shows techniques for controlling the engagement of atubed HYTREL® plastic extrudate with the core being enclosed. In U.S.Pat. No. 4,206,611, which issued on June 3, 1980 in the names of W. M.Kanotz et al., an extruded tubular covering is held out of contact withan advancing conductor until the extrudate becomes sufficientlyform-sustaining by suitable crystallization. Then, when the crystallizedinsulation is drawn down on the conductor, any tinsel burrs whichprotrude outwardly are compressed. This results in a conductor having acontinuously concentric insulation and a uniform wall thickness.

There are insulating operations other than these which are used to covertinsel conductors in which problems have developed because of theplastic which is extruded. For example, a low resistance cord mayinclude a plurality of conductors each comprising wires which arestranded together and insulated. Relatively high pressures are requiredto extrude some plastic materials such as the hereinbefore-mentionedHYTREL® plastic material. Such plastic materials have a high molecularweight and are polymerically branched, and normal pressure extrusiontechniques may cause dramatic melt viscosity and shear stress increasesthereby causing melt fracture.

Melt fracture of particular plastic materials during extrusion is astructural breakdown by fracture within a polymer melt where thecritical shear stress becomes abnormally independent of extrusion dieorifice size. The result is an insulation cover which is extremelyirregular and totally unacceptable.

And yet, these plastic materials such as HYTREL® elastomers have much tooffer. They generally are tough and mechanically resistant to many ofthe conditions encountered by insulated substrates in the field. It ishighly desirable to be able to take advantage of these benefits; but todo so, the problem of melt fracture must be overcome.

It should be clear, that there are several problems in the extrusion ofparticular plastic materials which must be addressed. Moreover,extrusion techniques need to be reexamined to find solutions to problemscaused during the covering of non-uniform substrates. Prior artextrusion technology seemingly lacks tooling which is capable ofextruding a substantially uniform, substantially concentric wall about asubstrate which is irregular or which includes intermittent overizedportions.

SUMMARY OF THE INVENTION

The foregoing needs have been met by the methods of this invention. Amethod of extruding a plastic material about a substrate includes thestep of advancing successive increments of length of a substrate througha core tube which is positioned in a cavity of a die and through a landof the die to an exit orifice. A flow passage for the plastic materialis provided through the cavity of the die and through the land. The flowpassage has a converging first portion between the tube and a wall ofthe die cavity which has an annular cross-section that decreases in areatoward an end of the tube to cause the minimum area of the first portionof the flow passage to be adjacent to the end of the tube. The end ofthe tube is spaced a predetermined distance from an entrance to the landto form a second portion of the flow passage having a predeterminedconfiguration. The plastic material is flowed through said passage withthe converging first portion causing the maximum pressure in the plasticmaterial to occur adjacent to the end of the tube. Also, the firstportion cooperates with the predetermined configuration of the secondportion to reduce substantially the pressure differential in the plasticmaterial between the second portion and the exit orifice.

The core tube and the die are arranged to form the second portion of theflow passage, which is called the gum space, such that the plasticmaterial expands into the second portion as it flows past the end of thetube. Because of the predetermined configuration of the second portionof the flow passage, the expanding plastic material does not completelyfill the second portion. As a result of the shifting of the location ofthe maximum pressure from the land to the end of the core tube, backflowof the plastic material toward the core tube and melt fracture ofparticular polymer materials are prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the present invention will be more readily understoodfrom the following detailed description of specific embodiments thereofwhen read in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view partially in section to show portions ofan extruder crosshead which includes tooling and associated facilities;

FIG. 2 is an enlarged view of a portion of the extruder crosshead ofFIG. 1 to show a hybrid arrangement for covering substrates;

FIGS. 3A and 3B are end section and perspective views, respectively ofstranded and tinsel cords which are read in the communications industryand which may be terminated with modular plugs;

FIG. 4 is an overall view of a manufacturing facility, in schematicform, embodying tooling for insulating substrates;

FIG. 5 is an enlarged view in section of a portion of a prior art tubingarrangement;

FIG. 6 is en enlarged view in section of a portion of a prior artpressure tooling arrangement;

FIG. 7 is an enlarged view of a portion of the core tube and a wall ofthe die cavity to show a restriction in the flow passage; and

FIG. 8 is an alternate embodiment of the arrangement shown in FIG. 2with provisions for permitting the passage of intermittentirregularities.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, there is shown an extruder crossheadapparatus, which is designated generally by the numeral 20 and whichincludes extrusion tooling of this invention that is designatedgenerally by the numeral 21. The apparatus 20 is adapted to extrude aninsulation cover of a plastic material over a substrate such as astranded wire conductor core, designated generally by the numeral 22(see FIG. 3A) or a tinsel conductor 23 of a cord, designated generallyby the numeral 26 (see FIG. 3B). While the invention is described andshown in terms of insulating conductors 24--24 which are strandedtogether or tinsel conductors 23--23, the principles of this inventionare applicable generally to the extrusion of insulative covers about asubstrate.

The apparatus 20 of this invention is specially suited to provide auniformly smooth insulative cover which is disposed tightly about astranded core. It is also specially suited to provide such a cover abouta tinsel conductor 23 notwithstanding the presence of intermittentoversize portions caused by splices.

Each of the insulated tinsel conductors 23--23 includes a nylonmulti-filament center core 32 about which a plurality of tinsel ribbons33--33, made typically from a Phosphor-bronze material, are wrappedspirally. An insulating cover 34 of suitable plastic material generallyis extruded over the tinsel ribbons 33--33 to form insulated tinselconductors 36--36. The insulated conductor 36 is disclosed and claimedin priorly identified U.S. Pat. No. 4,090,763 and the plastic materialwhich comprises the insulation 34 is a, relatively high molecularweight, branched polymer material. An insulation composition suitablefor constructing the cord 26 is available presently from the E. I.duPont de Nemours and Company, Inc. of Wilmington, Del. under the tradedesignation HYTREL® polyester elastomer material.

The tooling 21 of this invention is specially suited to the extrusion ofa relatively high molecular weight polymer about a substrate. Such apolymer is the hereinbefore-described one which is used in theproduction of cordage for telephone cords. As will be recalled,extrusion of the above-identified plastic material is characterized byrapid changes in melt viscosity and melt strength with slight variationsof polymer temperature. For particular polymers such as relatively highmolecular weight and/or branched polymers, the melt viscosity increasessignificantly as the pressure increases. These characteristics couldresult in non-uniform wall thickness and polymer flow pulsations unlesssuitable control is exercised.

The extension tooling 21 of this invention produces a product having theattributes of those of pressure extrusion and is capable ofaccommodating substrates having a non-uniform cross section along theirlengths. The characteristics of pressure extrusion are necessary in someapplications to provide relatively tight bundling of the strandedconductors in order to maintain acceptable electrical properties. In oneexample substrate, the outer diameter varied between 0.012 and 0.018inch with intermittent 0.024 inch splice sections. The required outerdiameter of the finished product was 0.037 inch. The intermittentoversized sections of the tinsel conductors 23--23 may come aboutbecause of splices.

Referring now to FIG. 4 of the drawings, there is shown a simplifiedschematic view of a system designated generally by the numeral 40 forproducing an insulated substrate in accordance with this invention. Thesystem 40 includes a supply 41 of a tinsel core, an accumulator 42, anextruder, designated generally by the numeral 43 including the crosshead20 for extruding the insulation cover 38 over the stranded core, acooling system 44, a capstan 46 and a takeup 47. The supply 41, theaccumulator 42, the cooling system 44, the capstan 46 and the takeup 47are all of conventional design and are well known in the art. Forinsulating a stranded wire core, individual wires may be fed fromindividual supply spools, and bundled together in a wire guide adjacentto the extruder crosshead 20.

Referring to FIG. 4 and again to FIGS. 1 and 2 of the drawings, it isseen that the extruder 43 includes a barrel 49 in which is mounted ascrew of the type for example shown in U.S. Pat. No. 3,579,608,incorporated by reference hereinto, which is rotated by suitable sourceof power (not shown) for the purpose of forcing the plastic materialthrough the extruder crosshead 20. The crosshead 20 in which is mountedthe tooling 21 comprises a body member 32 provided with an opening whichforms a continuation of the bore in the barrel 49 and which communicateswith a cylindrical bore 54 formed in the body member 52 transverselywith respect to the barrel.

A cylindrical tool holder 60 having a central bore 61 which extendscoaxially with respect to the bore 54 is removably mounted in the bodymember 52 by a back head nut 63 and an adapter nut 64. The tool holder60 supports a die 69 having a land 70 and mounts a core tube 71 in axialalignment with the die 69. Typically, the length of the land 70 asmeasured in a direction along a path of travel of the conductor is inthe range of 0.20 cm to 0.25 cm. The tool holder 60 is designed todeflect insulation material from a direction flowing downwardly asviewed in FIG. 1 to a direction flowing to the right around the coretube 71 and through the die 69 to form concentrically the coveringaround the substrate being advanced therethrough.

In a conventional tubing operation shown in FIG. 5, a plastic material76 is drawn down on a substrate 77 being advanced out of the extruder 43since the substrate is being advanced at a higher rate than that of theextrudate. The plastic melt emerging from the extruder 43 is referred toas the extrudate. See, for example, page 4 of Engineering Principles ofPlasticating Extrusion by Z. Tadmor and I. Klein published in 1970 byVan Nostrand Reinhold Co., and the hereinbefore identified Processing ofThermoplastic Materials, both of which texts are incorporated byreference hereinto. As is seen in FIG. 5, a core tube 78 has a portionwhich generally extends beyond or is flush with an exit orifice 79 of adie 81.

On the other hand, in a normal pressure extruder 82 which is shown inFIG. 6, a tip 85 of a core tube 83 is spaced a substantial distance froma land 84 of a die 86. For example, in a typical pressure extruder, thatdistance, designated L_(GS) and referred to as the length of a gum space87, may be as high as about 0.25 inch. On page 48 of the first editionof the Wire and Cable Coaters' Handbook published by E. I. DuPont in1968, a gum space length of 100-135 mils was disclosed to be best forthin wall wire coatings. This spacing is used to allow the flow of theplastic material to stabilize and the melt pressure to maximize beforeit enters the land 84 of the die 86. This creates a pressuredifferential between the gum space 87 and the orifice of the die 86.

Referring again to FIGS. 1 and 2, there is shown in detail the core tube71 and the die arrangement of the hybrid extrusion tooling 21 of thisinvention. The core tube 71 includes a stepped, tapered conical memberhaving an enlarged base portion 91, and conically shaped portions 92 and93 having successively reduced diameters but with outside walls thereofbeing generally converging toward a centerline 94 of the core tube 71.The core tube 71 is supported in the cavity 61 in the crosshead 20 suchthat a free end 98 of the portion 93 is spaced a distance L_(GS) fromthe beginning of the land 70 to form a gum space 99. This distance, aswill be recalled, is referred to as the length of the gum space 99. Ithas been found that a distance in the range of about 0.020 to about0.050 inch is suitable for the tooling 21 of this invention.

The core tube 71 is constructed with a tapered bore 102 extendingthrough the portions 91-93 and is defined by an inner surface 103.Typically an angle α between the inner surface 103 and the centerline ofthe core tube 71 is in the range of about 7° to 9°. The bore 102communicates with a relatively short length cylindrical bore 104 whichopens to the free end 98 of the core tube 71.

A useful ratio for comparing the hybrid extruder tooling 21 of thisinvention with that of prior art extruders is the ratio of the diameterof the core tube bore 104 with the diameter of the substrate to beinsulated. In the prior art, the ratio typically has been on in therange of 1.0-1.05 to 1. For the hybrid extruder, this ratio can be ashigh as 4 to 1 which of course renders the extruder capable of passinglarger oversize portions than was heretofore possible.

The die holder 60 is constructed with a cavity 106 which is defined by aside or bearing wall 107. The die 69 is supported in engagement with thesurface 107 and includes a cavity 108 that has a frustoconicalconfiguration and that converges at some predetermined angle toward theland or throat 70 adjacent an opening 109 of the die 69. Typically, theangle β formed between a line of generation of a wall 110 which definesthe cavity 108 and the centerline 94 of the core 71 is on the order ofmagnitude of 10° to 15°.

It should be observed from FIGS. 7 and 8 that the leading peripheraledge of the core tube 71 is beveled to form a flat portion 111 having awidth of about 0.020 inch. Moreover, the flat portion is spaced from thewall 110, which defines the cavity 108, a uniform distance that is inthe range of about 0.020 to 0.035 inch. The constant cross-section ofthe flow passage between the surface 110 and the beveled portion 111 ofthe core tube 71 creates a concentric annulus of stable polymer flow ata uniformly high velocity.

This arrangement of the flat portion 111 of the core tube 71 and the diecavity wall 110 provide a restriction 112 to plastic flow. Therestriction 112 is sufficient to cause the pressure of the plasticmaterial to attain a maximum value as it flows therethrough. The flowpassage between the cavity wall 110 and a frustoconical surface 113 ofthe core tube 71 gradually decreases to the restriction 112. As aresult, the velocity, melt viscosity and pressure in the plasticmaterial increase to a maximum within the restriction 112.

This is unlike a conventional pressure extruder. There, the distancebetween the tip of the core tube 71 and the cavity wall is on the orderof 0.100 inch and the greatest pressure occurs at the land of the die.In short, prior art tooling was designed not to restrict the flow in thevicinity of the leading end of the core tube. Also, as will be recalled,the length of the gum space 99 was substantially longer than in thetooling 21 of this invention. Use of the tooling 21 results in acontinuously concentric insulated conductor. This result had not beenattainable before in extrusion tooling having oversize core tubes topermit the passage of the enlarged substrate portions.

The tooling 21 of this invention defines a flow passage for the plasticmaterial. A first portion 116 is formed between the converging surfaces113 and 110 of the core tube 71 and the die cavity and terminates in therestriction 112. A second portion is the gum space 99 which is speciallyconfigured and which has the predetermined length L_(GS). The flowpassage also includes the land 70 of the die 69.

The plastic material is flowed along the first portion 116 of the flowpassage between the wall 110 of the die cavity 108 and the frustoconicalsurface 113 of the core tube 71 which are spaced apart in a convergingdirection. The first portion of the flow passage has an annularcross-section which decreases in area in a direction toward the free end98 of the core tube 71. The plastic material flows at an increasingvelocity to the end face of the core tube 71 and the pressure of thematerial on all sides of the portion of the core tube is balanced. Thesubstrate such as the stranded core 22 is guided from the core tube 71substantially in alignment with the cylindrical passage 109 of the die69 so that it is centered generally within the extrudate as it isadvanced out of the cylindrical bore and the insulation cover materialis extruded through the cylindrical passage in the die.

Beyond the end of the core tube 71, the plastic melt is flowed throughthe gum space 99 which ends at the entrance to the land 70 of the die69. In this second portion of its flow passage, the plastic melt isrestrained by only one surface, the inner surface 110 of the cavity 109.In the gum space 99, between the free end 98 of the core tube 71 and theentrance to the land, the plastic melt expands volumetrically as itsenses a drop in pressure. The pressure drop also causes the meltviscosity to decrease significantly as the plastic material recoversfrom its elastic deformation.

Since the configuration of the gum space 99 is significant to thetooling of this invention, it becomes important to characterize itrelative to that of other conventional tooling. One measure of itsconfiguration is the ratio of the length, L_(GS), of the gum space tothe inner diameter D_(i) of the die. Assuming a D_(i) of 0.038 inch, theratio for conventional tooling falls in a range of about 2.5-3.5 to 1.For the hybrid extruder of this invention, the ratio falls in a range ofabout 0.5-1.3 to 1 or simply 0.5 to 1.3 expressed as a number.

Another characterization can be had by a ratio in which the numerator isthe outer diameter OD_(cT) of the core tube 71 at its beveled free endless the outer dimension or diameter OD_(s) of the substrate and thedenominator is the length of the gum space. Assuming an OD_(s) of about0.02 inch and an OD_(cT) which may be as low as 0.03 inch for some knifeedge versions to a high of about 0.125, that ratio for conventionalpressure extruders is in the range of about 0.5 to 0.8, always lessthan 1. For the hybrid extruder of this invention, with a gum spacelength falling in the range of 0.02 to 0.05 inch and a core tube OD_(cT)of 0.07 to 0.09 inch, this ratio is in the range of 1.0 to 2.5, alwaysat least 1.

As a result of the configuration of the gum space 99, the plastic meltis directed toward the substrate at a velocity which is slightly lessthan that of the substrate and at only a slight angle thereto. Theplastic melt does not fill completely the gum space and meets thesubstrate just behind the land 70 of the die 69. The configuration andlength of the gum space portion of the flow passage is such thatpressure and viscosity drop significantly before the substrate is ableto impart drag forces to it and pull it through the die. This is unlikea conventional pressure extruder where the core tube is spaced from theland a distance which is sufficient to allow a pressure increase in thegum space and create a pressure differential that contributessignificantly to the flow through the die. With the arrangement of thisinvention, the differential pressure flow due to a pumping of theplastic has been reduced substantially. In one embodiment, the pressuredifferential is virtually negligible.

Inasmuch as the differential in pressure between the gum space at theentrance to the land and the die orifice may be substantiallynegligible, the plastic melt is moved through the die land by drag flowforces caused by its engagement with the substrate. The plastic meltcovers the substrate uniformly and smoothly. Because of the relativelylow gum space pressure, backflow of the plastic melt into the core tubeis prevented which could cause a problem if an oversize core tube 71were used to accommodate a spliced substrate such as that shown in FIG.8.

For relatively high molecular weight and/or branched polymer materialssuch as the hereinbefore-described HYTREL® plastic, the toolingarrangement of this invention prevents melt fracture. By substantiallyeliminating the differential pressure prior at the land 70 along withcreating volumetric expansion along with melt viscosity reduction pastthe point of greatest pressure, the shear stress within the land isreduced substantially over that of conventional tooling. The criticalshear stress within the polymer is reduced sufficiently to preventstructural breakdown of the polymer by fracture.

The restriction 112 that is created in the flow passage between thecavity wall 110 and the core tube 71 cooperates with the speciallyconfigured gum space 99 to provide the excellent results achieved withthe tooling 21 of this invention. It has been found that the restriction112 to the flow passage would not, without the specially configured gumspace, provide a relatively smooth insulative cover. This most likelyoccurs because the drag flow within the die 69 becomes dominant due tothe substantial reduction in the differential pressure flow. Bypositioning the tip of the core tube 71 adjacent to the land of the diein accordance with this invention, the drag flow is controlled relativeto the differential pressure flow and any impediment to a relativelysmooth surface is removed.

The product which is made by the methods and apparatus of this inventionis also unique, particularly that of a high molecular weight and/orbranched polymer insulation over a stranded core. Requirements for someof the more recently developed cords include relatively low electricalresistance and improved mechanical properties when terminated with amodular plug.

It has been found that the product of this invention has severalimportant properties. A cord constructed of such conductors exhibitsimproved resistance to bending and longer flex life at its entry intothe plug. The insulation cover is also capable of being pushed into theopenings of the plug without fraying the insulation. Moreover, cold flowand fracture of the insulation adjacent to strain relief systems in theplug have been eliminated.

For a conductor which is insulated with HYTREL® plastic in accordancewith this invention, the plastic surface has a matte finish. This causesthe conductor plastic to lock into a subsequently extruded plasticjacket. As a result, slippage between the conductors and the jacket isprevented and metal bands, which are clamped over the jacket for somecord uses, remain secured to the cord with substantially improvedresistance to pull.

Tubing is not a viable alternative for insulating stranded conductorsfor some uses. A tubed insulation does not lock in around the conductorswhereas the hybrid extruded plastic material is disposed tightly aboutand in the interstices of the stranded wires. The absence of suitablelock-in could cause problems with respect to termination and also allowsuck-back of the wires within the insulation.

It is to be understood that the above-described arrangements are simplyillustrative of the invention. Other arrangements may be devised bythose skilled in the art which will embody the principles of theinvention and fall within the spirit and scope thereof.

What is claimed is:
 1. A method of extruding a plastic material about asubstrate, said method including the steps of:advancing successiveincrements of length of a substrate through a core tube which ispositioned in a cavity of a die and through a land of the die which hasa circular cross-section to an exit orifice; providing a flow passagehaving a converging first portion which is formed between the core tubeand a wall of the die cavity and which has an annular cross-section thatdecreases in area toward an end of the core tube to cause the minimumarea of the first portion of the flow passage to be adjacent to the endof the core tube; spacing the end of the core tube which has a circularcross-section a distance from an entrance of the land to form a secondportion of the flow passage having a predetermined configuration, thedistance being such that a ratio having a numerator comprising thedifference between an outer diameter of the end of the core tube and anouter dimension of the substrate and having a denominator of thedistance is at least 1 and such that ratio of it to an inner diameter ofthe die is in the range of about 0.5 to 1.3; and flowing the plasticmaterial through the flow passage with the converging first portioncausing the maximum pressure in the plastic material to occur adjacentto the end of the core tube and cooperating with the predeterminedconfiguration of the second portion to cause a controlled expansion ofthe plastic material without filling the second portion of the flowpassage to thereby reduce substantially the pressure differential in theplastic material between the second portion and the exit orifice and toallow an inner diameter of the core tube to be substantially larger thanthe outer dimension of the substrate being advanced therethrough.
 2. Themethod of claim 1, wherein the minimum area of the first portion of theflow passage is caused to occur over a finite distance and through anannulus having a constant width.
 3. The method of claim 1, wherein theflow of the plastic material comprises a drag flow component which iscaused by its engagement with the advancing substrate and a differentialpressure flow caused by the difference in pressure between the landentrance and the exit orifice and wherein said differential pressureflow is substantially negligible.